The present invention relates to an audio signal noise reduction system and, more particularly, a noise reduction system capable of removing a pulse-like noise (referred to as xe2x80x9cpulse-like noisexe2x80x9d or xe2x80x9cnoisexe2x80x9d hereinafter) in a car radio, etc., which is caused by an ignition plug of an engine, an window open/close driving motor, etc.
FIG. 53 is a block diagram of a pulse-like noise reduction system in the prior art, which is set forth in Patent Application Publication (KOKAI) Sho 63-87026, for example. In FIG. 53, when receives an FM intermediate frequency signal of an FM receiver, an FM detector circuit 1 outputs a detected signal to a delay circuit 2 formed of an LPF (low-pass filter) which then delays the signal. Then, an output of the delay circuit 2 is supplied to a stereo demodulator circuit 5 via a gate circuit 3 and a level hold circuit 4. The detected signal is supplied to an HPF (high-pass filter) 6 to detect a noise. A noise signal passed through the HPF 6 is amplified by a noise amplifier 7 and then supplied to a noise detector circuit 8.
The noise detector circuit 8 is composed of a rectifier circuit which rectifies an output signal of the noise amplifier 7. An output of the noise detector circuit 8 is supplied to a waveform shaping circuit 9 and an integration circuit 10. The waveform shaping circuit 9 converts the output of the noise detector circuit 8 into a pulse with a predetermined time width and supplies the pulse to the gate circuit 3. The gate circuit 3, when driven by the pulse which is supplied from the waveform shaping circuit 9 to the gate circuit 3, is brought into its signal cut-off state. In this signal cut-off state, a delayed output level being detected immediately before the signal cut-off is held by the level hold circuit 4 and then supplied to the stereo demodulator circuit 5. Hence, generation of a spike due to sudden change in potential can be prevented. An integration circuit 10 outputs a direct current signal in answer to the noise level by smoothing the output of the noise detector circuit 8 and then feeds back it to the noise amplifier 7, whereby an AGC loop is constructed.
The delay circuit 2 is provided to compensate a time period ranging from a time point when the pulse-like noise is supplied to the HPF 6 to a time point when the gate circuit 3 is brought into its cut-off state. Since an Lch (left channel) signal and an Rch (right channel) which are balanced-modulated around (Lch+Rch)/2 by using a frequency of 38 kHz, as shown in FIG. 54, are input into the stereo demodulator circuit 5, Lch and Rch signals can be picked out separately from the stereo demodulator circuit 5 by virtue of the time division at 38 kHz, for example.
FIG. 55 is a view showing an operation of the noise reduction system in the prior art. Assume that the output signal of the FM detector circuit 1 is such a signal that includes the pulse-like noise (symbol A) shown in FIG. 55A, a high frequency component of the output signal of the FM detector circuit 1 is extracted by the HPF 6, so that a signal shown in FIG. 55B can be detected. An output signal of the HPF 6 is amplified by the noise amplifier 7, then rectified by the noise detector circuit 8, and then converted into the pulse with a predetermined time width, as shown in FIG. 55C, by the waveform shaping circuit 9. The gate circuit 3 brings a period of the pulse-like noise (referred to as a xe2x80x9cpulse-like noise periodxe2x80x9d or xe2x80x9cnoise periodxe2x80x9d hereinafter) of the signal which is delayed by a predetermined time by the delay circuit 2, as shown in FIG. 55D, into a cut-off state, as shown in FIG. 55E. The level hold circuit 4 removes the pulse-like noise contained in the original signal by holding the level, which is detected immediately before the cut-off, during the cut-off period of the output of the gate circuit 3, as shown in FIG. 55F.
Since the pulse-like noise reduction system in the prior art is constructed as mentioned above, the pulse-like noise can be rejected. However, if the signal has an amplitude to some extent, discontinuity of the signal occurs at the end of the holding period, as shown in FIG. 56B, because of preceding value holding process, i.e., because a value detected immediately before the start of the holding period is held. As a result, the noise cannot be satisfactorily removed.
In addition, if the signal contains a high frequency component, the signal becomes discontinuous at both the beginning and end of the holding period, as shown in FIG. 56C, because of the preceding value holding process. Therefore, in some cases the presence of the noise rejection process is conspicuous in hearing sense.
Moreover, since the preceding value holding process is carried out before the stereo demodulation, there has been such a problem that, if there is a difference between the signals on both channels, signal portions which are largely different from the beginning and end of the process are caused on one channel, as shown in FIG. 56D, because of the preceding value holding process, and others.
The present invention has been made to overcome the above problems, and it is an object of the present invention to provide a noise reduction system which is capable of removing pulse-like noises from an audio signal without fail after stereo demodulation, and also causing no discontinuity between removed regions and their preceding and succeeding regions even if the audio signal contains a high frequency component.
An audio signal noise reduction system according to the present invention comprises a noise detecting means for detecting a noise of an audio signal and outputting a detection signal indicating a start time and an end time of a noise period of the noise; a first filter means for extracting a low frequency component of the audio signal; a low frequency band interpolation means for polynomial-interpolating the noise period of the low frequency component being extracted; a second filter means for extracting intermediate and high frequency components of the audio signal; a means for suppressing a level of the noise period of the intermediate and high frequency components being extracted; and a signal synthesizing means for synthesizing the low frequency component, whose noise period is polynomial-interpolated, and the intermediate and high frequency components, the level of whose noise period is suppressed, to output the audio signal.
In this audio signal noise reduction system, since the low frequency component of the audio signal is extracted by the low-pass filter means and then the noise period from which the pulse-like noise is detected is polynomial-interpolated, the protruded interpolation signal is never derived and also the signal does not become extremely discontinuous before and after the noise period at all. In addition, since the intermediate and high frequency components of the audio signal are extracted by the intermediate/high-pass filter, then the level of the noise period of the intermediate and high frequency components is suppressed, and then the intermediate and high frequency components are synthesized with the low frequency component which is polynomial-interpolated, the audio signal from which the noise is completely removed can be derived.
Further, since the noise period is interpolated and suppressed independently every channel after the stereo demodulation, no discrepancy is caused before and after the noise period even if there is difference between signals over a plurality of channels.
An audio signal noise reduction system comprises a noise detecting means for detecting a noise of an audio signal and outputting a detection signal indicating a start time and an end time of a noise period of the noise; a first filter means for extracting a low frequency component of the audio signal; a limiter means for limiting a degree of an inclination of a segment serving as a start point of the noise period of the low frequency component being extracted; a means for polynomial-interpolating the noise period of the low frequency component in which the degree of the inclination of the segment serving as the start point of the noise period is limited; a second filter means for extracting intermediate and high frequency components of the audio signal; a means for suppressing a level of the noise period of the intermediate and high frequency components being extracted; and a signal synthesizing means for synthesizing the low frequency component, whose noise period is polynomial-interpolated, and the intermediate and high frequency components, the level of whose noise period is suppressed, to output the audio signal.
According to the above configuration, the polynomial interpolation is carried out after the low frequency component is extracted from the audio signal by the low-pass filter and then the degree of the inclination of the segment which is defined by two respective preceding and succeeding points of the noise period serving as the polynomial interpolation of the noise period is limited by the limiter means. Therefore, unless the cut-off characteristic of the LPF which extracts the low frequency component is set to be steep, the protruded interpolation signal is never derived and no prominent discontinuity is caused at the start and end points of the noise period. In addition, since the intermediate frequency component is extracted from the audio signal by the intermediate/high-pass filter, then the level of the noise period of the intermediate and high frequency components is suppressed, and then the intermediate and high frequency components are synthesized with the low frequency component which is polynomial-interpolated, the audio signal from which the noise is completely removed can be derived.
An audio signal noise reduction system comprises a noise detecting means for detecting a noise of an audio signal and outputting a detection signal indicating a start time and an end time of a noise period of the noise; a linear interpolation means for linear-interpolating the noise period of the audio signal; a first filter means for extracting a low frequency component of the audio signal being linear-interpolated; a means for polynomial-interpolating the noise period of the low frequency component being extracted; a second filter means for extracting intermediate and high frequency components of the audio signal being linear-interpolated; a means for suppressing a level of the noise period of the intermediate and high frequency components being extracted; and a signal synthesizing means for synthesizing the low frequency component, whose noise period is polynomial-interpolated, and the intermediate and high frequency components, the level of whose noise period is suppressed, to output the audio signal.
According to the above configuration, since the low frequency component is extracted and then the polynomial interpolation is executed after the noise period of the audio signal has been linear-interpolated previously, the inclination of the segment defined by two preceding and succeeding points of the noise period serving as the start points of the polynomial interpolation can be made small in comparison with the case where the low frequency component is extracted by the LPF while containing the pulse-like noise as it is. Therefore, the protruded interpolation signal is never generated and no remarkable discontinuity of the signal is caused at the start and end of the noise period. In addition, since the intermediate frequency component is extracted from the audio signal by the intermediate/high-pass filter, then the level of the noise period of the intermediate and high frequency components is suppressed, and then the intermediate and high frequency components are synthesized with the low frequency component which is polynomial-interpolated, the audio signal from which the noise is completely removed can be derived.
An audio signal noise reduction system comprises a noise detecting means for detecting a noise of an audio signal and outputting a detection signal indicating a start time and an end time of a noise period of the noise; a linear interpolation means for linear-interpolating the noise period of the audio signal; a first filter means for extracting a low frequency component of the audio signal being linear-interpolated; a limiting means for limiting a degree of an inclination of a segment serving as a start point of the noise period of the low frequency component being extracted; a means for polynomial-interpolating the noise period of the low frequency component in which the degree of the inclination of the segment serving as the start point of the noise period is limited; a second filter means for extracting intermediate and high frequency components of the audio signal being linear-interpolated; a means for suppressing a level of the noise period of the intermediate and high frequency components being extracted; and a signal synthesizing means for synthesizing the low frequency component, whose noise period is polynomial-interpolated, and the intermediate and high frequency components, the level of whose noise period is suppressed, to output the audio signal.
According to the above configuration, since the low frequency component is extracted and then the polynomial interpolation is executed after the noise period of the audio signal has been linear-interpolated previously, the inclination of the segment defined by two preceding and succeeding points of the noise period serving as the start points of the polynomial interpolation can be made small in comparison with the case where the low frequency component is extracted by the LPF while containing the pulse-like noise as it is. Therefore, the protruded interpolation signal is never generated and no remarkable discontinuity of the signal is caused at the start and end of the noise period. In addition, since the intermediate frequency component is extracted from the audio signal by the intermediate/high-pass filter, then the level of the noise period of the intermediate and high frequency components is suppressed, and then the intermediate and high frequency components are synthesized with the low frequency component which is polynomial-interpolated, the audio signal from which the noise is completely removed can be derived.
An audio signal noise reduction system comprises a noise detecting means for detecting a noise of an audio signal and outputting a detection signal indicating a start time and an end time of a noise period of the noise; a linear interpolation means for linear-interpolating the noise period of the audio signal; a first filter means for extracting a low frequency component of the audio signal being linear-interpolated; a second filter means for extracting intermediate and high frequency components of the audio signal being linear-interpolated; a means for suppressing a level of the noise period of the intermediate and high frequency components being extracted; and a signal synthesizing means for synthesizing the low frequency component being extracted and the intermediate and high frequency components, the level of whose noise period is suppressed, to output the audio signal.
According to the above configuration, the low frequency component is extracted by the LPF after the noise period of the audio signal has been linear-interpolated in advance, and then such low frequency component and the intermediate and high frequency components which are extracted from the linear-interpolated audio signal and whose noise period is suppressed in level are synthesized. Therefore, smoothness of the signal lacks rather than the case where the noise period of the low frequency component is polynomial-interpolated, but the audio signal from which the noise is completely removed can be derived.
The second filter means for extracting intermediate and high frequency components of the audio signal is composed of a delay means for delaying the audio signal by the same amount as that in the first filter means, and a subtracting means for subtracting the low frequency component extracted by the first filter means from the audio signal being delayed.
According to the above configuration, the second filter means for extracting intermediate and high frequency components can be accomplished with a simple configuration.
An audio signal noise reduction system further comprises a third filter means for extracting an intermediate frequency component from the intermediate and high frequency components which are extracted by the second filter means; a fourth filter means for extracting a high frequency component from the intermediate and high frequency components; a means for suppressing levels of noise periods of the intermediate frequency component and the high frequency component being extracted respectively; and a signal synthesizing means for synthesizing the low frequency component, whose noise period is polynomial-interpolated, and the intermediate frequency component and the high frequency component, the levels of whose noise periods are suppressed, to output the audio signal.
According to the above configuration, the third and fourth filter means for separating the intermediate and high frequency components being extracted by the second filter means into the intermediate frequency component and the high frequency component can be implemented with a simple structure.
The means for suppressing a level of the noise period of the intermediate and high frequency components being extracted by the second filter means, includes a muting means for attenuating the noise period of the intermediate and high frequency components, and a fade-out/in means for fading out the intermediate and high frequency components immediately before the noise period and fading in them immediately after the noise period, otherwise an ON/OFF means for turning OFF the intermediate and high frequency components at a beginning of the noise period and turning ON them at an end of the noise period.
According to the above configuration, since the rejecting means can be employed selectively according to the noise component contained in the intermediate and high frequency components, the noise can be rejected without damage of the audio signal.
The means for suppressing the noise period of the intermediate frequency component which is extracted by the third filter means, includes a fade-out/in means for fading out the intermediate frequency component immediately before the noise period and fading in it immediately after the noise period, and an ON/OFF means for turning OFF the intermediate frequency component at a beginning of the noise period and also turning ON it at an end of the noise period, otherwise a level down means for suppressing a level of the noise period, and the means for suppressing the level of the noise period of the high frequency component being extracted by the fourth filter means includes a fade-out/in means for fading out the high frequency component immediately before the noise period and fading in it immediately after the noise period.
According to the above configuration, the noise components contained in the noise period of the intermediate frequency component and the high frequency component respectively can be rejected effectively.
Fade-out/in characteristics of the fade-out/in means are set like a saturation curve which saturates from a pass-band region toward a cut-off region respectively.
According to the above configuration, discontinuity between the noise period and the signals at the start and end of the noise period can be reduced.
An audio signal noise noise reduction system comprises a noise detecting means for detecting a noise which is mixed into an audio signal; a first delay means for providing a delay amount generated in the noise detecting means to the noise signal; a first filter means for extracting a low frequency component of the audio signal being output from the first delay means; a low frequency band interpolation means for polynomial-interpolating a period of an output of the first filter means, from which the noise is detected by the noise detecting means; a second delay means for providing a delay amount generated in the low frequency band interpolation means to the audio signal output from the first delay means; a second filter means for extracting a high frequency component of the audio signal output from first delay means; a memory means for storing an output of the second filter means; a high frequency band interpolation means for reading a high frequency component of the period, from which the noise is detected by the noise detecting means, from the memory means and polynomial-interpolating it; and a signal synthesizing means for exchanging the period of the audio signal output from the second delay means, from which the noise is detected by the noise detecting means, with a synthesized signal of an output signal of the low frequency band interpolation means and an output signal of the high frequency band interpolation means.
A low frequency band interpolation signal is generated by extracting the low frequency component of the audio signal by using the low-pass filter and then applying the polynomial interpolation of the low frequency component to the period from which the pulse-like noise is detected. A high frequency band interpolation signal is generated by extracting the high frequency component of the audio signal by using the high-pass filter, then storing it into the memory sequentially for a predetermined period, and then reading the past preceding signal from the memory over the period from which the pulse-like noise is detected. Finally, the low frequency band interpolation signal and the high frequency band interpolation signal are synthesized, and this synthesized signal is exchanged with the original audio signal over the period from which the pulse-like noise is detected.
An audio signal noise noise reduction system comprises a noise detecting means for detecting a noise which is mixed into an audio signal; a first delay means for providing a delay amount generated in the noise detecting means to the noise signal; a first filter means for extracting a low frequency component of the audio signal being output from the first delay means; a low frequency band interpolation means for polynomial-interpolating a period of an output of the first filter means, from which the noise is detected by the noise detecting means; a second delay means for providing a delay amount generated in the low frequency band interpolation means to the audio signal output from the first delay means; a frequency analyzer means for analyzing frequency components of the audio signal output from first delay means; a high frequency component generating means for detecting a single prominent frequency or a plurality of prominent frequencies within a frequency range, which is cut off by the first filter means, of an output signal of the frequency analyzer means immediately before the period, from which the noise is detected by the noise detecting means, and below a Nyquist frequency and then generating a sine wave of the frequency or frequencies; and a signal synthesizing means for exchanging the period of the audio signal output from the second delay means, from which the noise is detected by the noise detecting means, with a synthesized signal of an output signal of the low frequency band interpolation means and an output signal of the high frequency component generating means.
The low requency band interpolation signal is generated by extracting the low frequency component of the audio signal by using the low-pass filter and then applying the polynomial interpolation of the low frequency component to the period from which the pulse-like noise is detected. According to the result of the frequency analysis of the audio signal, the high frequency band interpolation signal is generated by detecting the single prominent frequency or a plurality of prominent frequencies within the frequency range, which is cut off by the low-pass filter means, immediately before the period, from which the pulse-like noise is detected, and below a Nyquist frequency and then generating a sine wave of the frequency or frequencies. Lastly, the low frequency band interpolation signal and the high frequency band interpolation signal are synthesized, and this synthesized signal is exchanged with the original audio signal over the period from which the pulse-like noise is detected.